Speed sensor assemblies for vehicle wheels are used to monitor the speed of the wheels for various purposes such as for implementing an automatic braking system (ABS), and/or as input to an engine or transmission controller. Typical speed sensor assemblies have included those used with capped wheel bearing assemblies, and those used with wheel bearing assemblies that use an inboard seal, but no bearing cap. Sensor assemblies integrated within a capped wheel bearing assembly typically have a magnetic encoder, often referred to as a tone ring, which rotates with the rotating part of the wheel assembly, and a stationary sensor spaced from the tone ring. Both of these components are sealed from the external environment by the bearing cap and one or more seals. Protecting the bearing, the tone ring and the sensor from the environment can be advantageous. However, servicing sensors integrated within a capped wheel assembly may require disassembly of the entire wheel assembly, and thus can require replacement of the entire wheel assembly even if only the sensor component may actually need replacement.
Speed sensors that are not sealed and capped within the bearing assembly have the advantage of easy removal for servicing. However, these designs typically have an exposed inboard seal and tone ring. This presents design challenges, as exposure to the environment can make it difficult to maintain a gap of a specified dimensional range between the sensor and the tone ring. Placing just the tone ring within a capped wheel assembly and positioning the sensor externally to the bearing cap to read the tone ring through the cap has the advantage of easy access to the sensor. However, such a design requires either that the sensor extend through the cap, thus diminishing the sealing effectiveness of the cap, or requires the use of a less precise tone ring. The latter is due to the increased distance between the sensor and the tone ring due to the bearing cap in between. The increased distance requires the use of a tone ring with fewer magnetic pole pairs in order for the pole pairs to be of a size that creates a sufficient magnetic field to be read by the sensor through the cap.